This invention relates to a method of controlling an automatic transmission equipped with a lock-up clutch and having a torque converter. More particularly, the invention relates to a method of controlling changeover of the lock-up clutch when speed ranges (gear ranges) in the transmission are shifted.
A conventional method of controlling a lock-up clutch (direct-coupling clutch or high-gear clutch) in an automatic transmission of the above-described type is disclosed in the specification of Japanese Patent Publication (KOKOKU) No. 60-32063 and will now be described in connection with FIGS. 8 and 9, which are diagrams illustrating examples of gear shift lines and lock-up clutch changeover lines in the D range.
In FIG. 8, the solid lines 1st.fwdarw.2nd, 2nd.fwdarw.3rd are up-shift lines from 1st gear range to 2nd gear range and from 2nd gear range to 3rd gear range, respectively. The dashed lines 1st.fwdarw.2nd, 2nd.fwdarw.3rd are down-shift lines from 2nd gear range to 1st gear range and from 3rd gear range to 2nd gear range, respectively. In FIG. 9, the solid line OFF.fwdarw.ON and dashed line OFF.fwdarw.ON in 2nd gear range represent a changeover line in which the lock-up clutch is switched from disengaged to engaged state in 2nd gear range, and a changeover line in which the lock-up clutch is switched from engaged to disengaged state in 2nd gear range, respectively. The solid lines OFF.fwdarw.ON and dashed line OFF.fwdarw.ON in 3rd represent a changeover line in which the lock-up clutch is switched from disengaged to engaged state in 3rd gear range, and a changeover line in which the lock-up clutch is switched from engaged to disengaged state in 3rd gear range, respectively.
In the conventional control method, as is evident from FIGS. 8 and 9, the arrangement is such that the lock-up clutch operates in each gear range from 2nd gear range onward in the throttle opening range of 0 to 100% in order to improve fuel consumption. When the vehicle is propelled forward to start from rest in the ordinary manner, the shifting process is such that an up-shift is made from 1st to 2nd, the lock-up clutch is engaged and disengaged in 2nd, an up-shift is then made from 2nd to 3rd, and the lock-up clutch is engaged in 3rd. If the automatic transmission has four forward speed ranges inclusive of overdrive, the shifting process further includes releasing the lock-up clutch, up-shifting from 3rd to O/D and then engaging the lock-up clutch. Thus, the engagement and release of the lock-up clutch is repeated a number of times in a short period.
Engaging and disengaging the lock-up clutch in this manner is accompanied by gear-shift shock that results in driver discomfort, which becomes more pronounced at lower throttle openings. The reason for this is that since vehicle acceleration is low at lower throttle openings, the change in acceleration attendant upon the engagement or disengagement of the lock-up clutch becomes relatively large with respect to vehicle acceleration and is easily sensed by the driver.